The use of membranes for separation processes is well known. Certain carbon membranes are particularly useful for the separation of fluids, especially gases such as oxygen and nitrogen.
The membranes may be fabricated in various geometrical configurations, such as sheet formed membranes and hollow fibers. The membranes may be symmetrical, asymmetrical, single-component or composite.
Carbon membranes have superior selectivities and productivities for many separations. However, a major problem with these membranes has been their vulnerability to the effects of water vapor and other condensible agents and impurities such as oils or other hydrocarbon compounds. For example, humidity levels well below 100% relative humidity are sufficient to significantly impair the performance of the carbon membrane. Small amounts of oil or other hydrocarbons can also significantly impair the performance of the membrane.
In order to reduce the humidity of the fluid to be permeated, the fluid may be treated with dehumidifying agents. This typically involves the use of large, expensive equipment. Such equipment is also prone to failure. In addition, other condensible agents and impurities may be removed from the fluid to be permeated by various filtration, separation or extraction techniques. These measures may also involve the use of large, expensive equipment and are often not successful.
It is known to prepare composite membranes and/or post treat membranes with materials that seal or heal defects or improve the stability of the membrane. For example, U.S. Pat. Nos. 3,616,607 and 3,775,303 exemplify gas separation membranes having superimposed membranes on a porous support.
U.S. Pat. No. 4,230,463 deals broadly with the post treatment of fluid separation membranes. It describes a wide variety of membranes for liquid and gas separations, particularly a multicomponent membrane where the separation properties of the membrane are principally determined by the porous separation membrane as opposed to the material of the coating. The coating cures defects in the surface of the membrane. U.S. Pat. No. 4,767,422 also discloses a method of posttreating composite membranes to cure defects in the thin separation layer. U.S. Pat. No. 4,728,345 describes a multicomponent membrane for gas separation having a polyphosphazene coating in occluding contact with a porous separation membrane for the purpose of improving stability of the membrane when exposed to aromatic and aliphatic hydrocarbons contained in a gaseous mixture.
EPO Patent Application 0,337,499 discloses a gas separation membrane with a covering layer formed from a selective film. The covering layer is made from a polymer having a critical surface tension not larger than 30 dynes/cm, such as poly-4-methylpentene-1, fluorinated alkyl methacrylate and polymethyl fluorinated alkyl siloxane.
U.S. Pat. No. Re. 33,273 describes a method of improving the characteristics of separatory membranes by the deposition of a fluorinated amphiphilic compound in an oriented layer on the surface of the membrane so as to increase membrane selectivity and counteract membrane surface properties leading to fouling caused by colloidal materials.
The prior art references do not, however, teach a polymeric membrane treatment for reducing the adverse effects of impurities on the performance of carbon membranes. A carbon membrane is, therefore, needed with good permeation properties and significant resistance to the effects of water vapor and other condensible agents and impurities. The inventive fluid separation membrane is a composite carbon membrane which retains high selectivities for fluid separations and is significantly more resistant to the adverse affects commonly observed in environments having high humidity or other impurities.